This invention relates to the fields of lasers and optics.
A frequent problem in the performance of solid state lasers is optical damage to the dielectric coatings forming the output coupler. Such multilayer dielectric-film coatings are generally the weakest element in a laser system, and typically fail at intensities below 10 GW/cm2 or fluences below 5 J/cm2. In high-gain pulsed lasers, the optical intensity at the output coupler is often larger than at other surfaces, making the output coupler a common source of problems.
In contrast to dielectric films, there are many bulk optical materials with a damage threshold in excess of 100 GW/cm2. As a result, polished etalons made from dielectric materials, such as quartz or sapphire, with highly parallel faces are often used as the output mirrors for pulsed high-power lasers. That is, the lasers are operated with a 100% mirror on one end and a polished etalon a few millimeters or a centimeter thick, generally with no additional coatings, as the output coupler on the other end. Since these lasers typically have large round-trip gains, they operate best with low-reflectivity output mirrors, and the uncoated dielectric etalon provides a simple way of achieving the necessary output mirror reflectivity. These uncoated etalons are simple to fabricate and can have very high optical-damage thresholds.
If a bulk etalon, as described above, is used in a solid-state laser, at least one endxe2x80x94the output endxe2x80x94of the solid-state gain medium must be treated to eliminate reflections at the solid-to-air interface. This could be done by depositing a dielectric antireflection coating on the gain medium, or by cutting the gain medium at Brewster""s angle. The use of such a dielectric coating can result in a lower threshold for optical damage. Cutting the gain medium at Brewster""s angle complicates the fabrication of the device and can lead to poorer performance.
In accordance with the invention, an output coupler is formed of two bodies of bulk material separated by a fluid-filled gap between the highly parallel faces of the bodies. Preferably, the bodies are formed of a high-damage-threshold material such as rutile (TiO2) and the spacing between the bodies is an odd multiple of one-quarter wavelength apart to achieve maximum reflectivity.